When laying a pipeline at sea one of two methods is commonly used: either the “S” laying method or the “J” laying method. The two methods are named in accordance with the general shape adopted by the pipeline during laying. In “S” laying, the pipeline leaves the vessel at little or no inclination to the horizontal, adopts a steeper inclination in the water and then returns to a generally horizontal disposition on the seabed. In “J” laying, the pipeline leaves the vessel at a steep or vertical inclination and the inclination steadily reduces until the pipeline is in a generally horizontal disposition on the seabed. As interest in laying pipeline in deep water has increased, so “J” laying has become more attractive because the pipeline naturally adopts a vertical or near-vertical orientation far from the seabed. “J” laying is not, however, preferred in shallower water where the natural path of the pipeline is only ever inclined at a shallow angle as it passes to the seabed and “S” laying is therefore advantageous.
“S” laying can be employed in deep water provided the pipeline being laid can be supported from the vessel until it has reached a relatively steep inclination to the horizontal. That, however, requires a considerable length of support, because the radius of bending to which the pipeline can be subjected is limited, especially in the case of large diameter pipelines.
One approach to providing an “S” laying vessel includes providing a semisubmersible vessel with twin keels. For example, U.S. Pat. No. 4,257,718 shows such an arrangement. Another approach to providing an “S” laying vessel involves providing a monohull vessel as shown, for example, in U.S. Pat. No. 5,823,712. An advantage of a twin keel semisubmersible vessel is that it can provide a relatively roll-free environment for the pipe laying and also considerable amounts of space to either side of a central pipe laying path on the vessel (also known to those skilled in the art as “the firing line”). The extra breadth of such a vessel and its relatively large draught are, however, disadvantages compared to a monohull vessel.
When laying small diameter pipes it is sometimes preferred to provide a long length of prefabricated pipeline on a reel and to lay the pipeline by unwinding the reel, but especially for large diameter pipelines it is usual to form the pipeline from discrete lengths of pipe each typically about 12 m long. In this case it is desirable to weld individual lengths of pipe into prefabricated pipe lengths (also referred to herein as “jointed pipe sections”) consisting of, say, two, three or four individual lengths of pipe and then to weld the prefabricated pipe lengths to the end of the pipeline as it is being laid. Such a procedure enables the speed of laying to be increased over that which would apply if each individual pipe length were welded in turn to the end of the pipeline as it is laid. Thus the vessel is required to accommodate welding stations and other facilities for preparing prefabricated pipe lengths. Such workstations are desirably placed at intervals corresponding to one prefabricated pipe length; it is then possible to feed out the pipeline one prefabricated length at a time and each workstation can then be positioned at a respective joint between pipe lengths. The more stations that can be provided along the pipelaying path, the less work has to be done at each station and therefore the faster can be the overall rate of pipelaying.
In WO 2008/107186 a monohull vessel is described which is able to “S” lay pipelines in a particularly advantageous and flexible manner. In that vessel a pivotally mounted internal ramp is provided at the end of the pipelaying path on the vessel and guides the pipeline into the water. Optionally, an external ramp or stinger is connected to the downstream end of the internal ramp to guide the pipeline to a steeper inclination as it passes downwardly away from the vessel and towards the seabed.
When S-laying a pipeline it is common practice to provide a stinger where the pipeline leaves the vessel to control the inclination of the pipeline and the bending of the pipeline as it leaves the vessel. Such an external ramp may be pivotally mounted so that its inclination can be adjusted according to the inclination at which the pipeline is desired to leave the ramp. An actuator of some kind is connected between the vessel and a position on the external ramp downstream from its pivotal mounting, to pivot the ramp about its mounting. The desired inclination may vary according to the depth of the water in which the pipeline is being laid and the flexibility of the pipeline which in turn may be affected by the material of the pipeline and the diameter and wall thickness of the pipeline. If a long external ramp is provided, then the distal end of the ramp tends to be some distance from the vessel. Thus, it is not very practical to form the external ramp of two or more ramps because of the difficulty of controlling pivoting of the most downstream ramp at some distance from the vessel.
USRe27420 describes a pipelaying vessel with a plurality of external ramps pivotally connected in series. A worm gear is provided between each pair of adjacent ramps, which controls the relative angle between the adjacent ramps. Use of a worm gear helps the ramps to maintain their relative positions even when power to the actuator driving the gear is switched off. There is, however, no system for positively locking the ramps in any relative positions.
It is an object of the invention to provide a vessel for laying a pipeline and a method of laying a pipeline which offers further advantages over the vessels and methods described above.
It is a further object of the invention to provide an external ramp assembly for laying a pipeline from a vessel, to a pipelaying vessel including such a ramp assembly and to a method of laying a pipeline using such a ramp assembly, in which one or more of the problems referred to above are overcome.
In a more particular aspect the invention seeks to provide certain improvements to and developments of the vessel that is the subject of WO 2008/107186, the disclosure of which is incorporated herein by reference, but it should be understood that the invention is also applicable to vessels of other designs.